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Commitments, Guarantees and Contingencies - Environmental Matters (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) MW	Dec. 31, 2017 USD ($)
NOV Consent Decree [Member]
Environmental Matters [Line Items]
Consent Decree Wind Energy Generation Addition (MW) | MW	200
Ozone NAAQS [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	$ 0.0
NO2 NAAQS [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	0.0
Clean Water Act - Aquatic Organisms [Member] | Maximum [Member] | Minnesota Power [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	$ 15.0
Coal Combustion Residuals [Member] | Minnesota Power [Member]
Environmental Matters [Line Items]
Expected Period for Costs of Compliance	15 years
Coal Combustion Residuals [Member] | Minimum [Member] | Minnesota Power [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	$ 65.0
Coal Combustion Residuals [Member] | Maximum [Member] | Minnesota Power [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	120.0
Manufactured Gas Plant [Member] | SWL&P [Member] | Superior, WI [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance (Accrued)	$ 7.0	$ 8.0
Expected Period to Incur Remediation Costs	four years

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Capacity of a generating facility, unit, or project in megawatts.

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Total costs accrued as of the balance sheet date for environmental loss contingencies.

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-Name Accounting Standards Codification
-Topic 450
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